The present invention relates to heat sensitive electrical switching devices and, more particularly, to snap action thermostatic electrical switches of the type which is small enough to be incorporated in an electrical device to protect the device from overheating.
It is desirable to protect electrical devices, such as motors, generators, and transformers, from the effects of overheating. While power supply line circuit breakers provide protection from excessive currents for such electrical devices, circuit breakers do not protect against overheating which may occur during continuous operation of a device at a current level which is not excessive. To provide adequate thermal protection for an electrical device, it is necessary that a thermally responsive protective switching arrangement be placed within the device to monitor the temperature of the device. In order for a thermostatic switch to be positionable within an electrical device, such as in the windings of an electric motor, it is necessary that the thermostatic switch be relatively small in size, but in fabricating such a miniaturized thermostatic switch, accurate positioning of the switch elements is difficult to achieve.
In several switch constructions, such as shown in U.S. Pat. No. 3,213,246, issued Oct. 19, 1965, to Duval, and U.S. Pat. No. 3,453,577, issued July 1, 1969, to D'Entremont, a switch casing is formed from a single piece of metal which is drawn into the desired elongated casing shape. Assembly of the switch elements within the switch casing then must necessarily be accomplished in a series of operations during which it is not possible to see the orientation of the switching elements. Such a "blind" assembly procedure is time consuming and can result in misalignment of switch elements.
Other switch constructions are known in which the casing for the thermostatic switch is formed of two separate pieces of material. U.S. Pat. No. 3,430,177, issued Feb. 25, 1969, to Audette discloses a thermostatic switch having a two-piece casing. The casing components are, however, joined either with a thermosetting adhesive or by crimping. U.S. Pat. No. 3,622,930, issued Nov. 23, 1971, to D'Entremont, discloses a motor protector switch having a two-part casing, which parts are joined in a crimp operation. It will be appreciated that such casing arrangements may not provide an adequately sealed casing and, also, when switch components are mounted on both portions of the casing, assembly of the casing results in a blind orientation of the switch components. Additionally, if components are mounted on both portions of the casing, it may be necessary to keep these casing portions electrically isolated to assure proper switching operations.
It is seen, therefore, that there is a need for a simple, easily assembled thermostatic electrical switch which is small in size and which provides reliable thermal switch actuation.